Can I set max render time instead of samples?
There is no such an option for it now (Good feature for sure). The similar way is to enable progressive refine in your Scene Performance panel.

It will treat all render area as a chunk, which give you the ability to manually stop at the point you think your image is well enough.
Here is the official description:

Progressive Refine
Instead of rendering each tile until it has
finished every sample, refine the whole image progressively. Note that
progressive rendering is slightly slower than tiled rendering, but
time can be saved by manually stopping the render when the noise level
is low enough.
For rendering animations it is best to disable this feature, as
stopping a frame early is not possible.

Note: If you found out that Progressive Refine check box is grey out. Then check if you are using Denoising option in Scene Render Layer. You cannot use them simultaneously.